Specialty fluoroelastomers made from copolymers of tetrafluoroethylene (TFE), propylene (P), and optionally vinylidene fluoride (VF2) (i.e. TFE/P dipolymers or VF2/TFE/P terpolymers) are often utilized in applications wherein resistance to alkaline fluids and other high pH chemicals is critical. The TFE/P dipolymers have the best resistance to alkaline fluids. Terpolymers containing more than about 10 wt. % vinylidene fluoride units generally do not have significantly better alkaline fluid resistance than do conventional fluoroelastomers made from copolymers of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene.
In order to fully develop physical properties such as tensile strength, elongation, and compression set, elastomers must be cured, i.e. crosslinked. In the case of fluoroelastomers, this is generally accomplished by mixing uncured polymer (i.e. fluoroelastomer gum) with a polyfunctional curing agent and heating the resultant mixture under pressure, thereby promoting chemical reaction of the curing agent with active sites along the polymer backbone or side chains. Interchain linkages produced as a result of these chemical reactions cause formation of a crosslinked polymer composition having a three-dimensional network structure. Commonly used curing agents for fluoroelastomers include difunctional nucleophilic reactants, such as polyhydroxy compounds. Alternatively, peroxidic curing systems containing organic peroxides and unsaturated coagents, such as polyfunctional isocyanurates, may be employed.
In many cases, polyhydroxy and peroxide cure processes or curing agent formulations are unsatisfactory when used to crosslink these specialty fluoroelastomers. For example, it is known to cure elastomeric VF2/TFE/P terpolymers with either peroxide (U.S. Pat. No. 4,910,260) or polyhydroxy (U.S. Pat. Nos. 4,882,390 and 4,912,171) cure systems. However, when such compositions are cured using a polyhydroxy compound, the cured products may exhibit undesirably high compression set. In fact, such specialty fluoroelastomers which contain less than about 10 wt. % copolymerized units of vinylidene fluoride show little to no cure response with polyhydroxy cure formulations.
The peroxide cures disclosed in U.S. Pat. No. 4,910,260 are undesirable because the curable compositions which are initially formed are extremely scorchy and would therefore be unsuitable for many commercial processes.
It would be particularly desirable to have an improved specialty fluoroelastomer that is resistant to alkaline fluids and which readily crosslinks with polyhydroxy cure systems to form cured articles having good tensile properties and compression set resistance.